A method of the type mentioned above is described in European Patent No. EP 1 172 541 A1. In the fuel injection device described, a valve element is provided in the form of a valve needle that can be opened or closed hydraulically by a pressure in a control chamber. The pressure in the control chamber is in turn influenced by a switching valve that is coupled to a piezoelectric actuator via a hydraulic coupler.
In addition, in a commercially available fuel injection device, the valve element is coupled to the piezoelectric actuator immediately (i.e., without the intermediate connection of a switching valve), likewise via a hydraulic coupler. Here, during the charging and discharging of the piezoelectric actuator, either the voltage curve of the piezoelectric actuator can be predetermined or a current curve is predetermined, which then results in a desired voltage at the end of the charging or discharging process. In the latter case, the predetermined current profile can additionally be scaled by a superposed voltage regulator, so that at least the voltage levels at the end of the charging or discharging process can be set by a closed control circuit.
Here, however, the voltage gradient cannot be set arbitrarily. On the one hand, it is limited by the maximum current of an output stage that controls the piezoelectric actuator, and on the other hand it is limited by the fact that when the voltage gradient is too high the danger exists that the resonance of the piezoelectric actuator will be excited, which can result in destruction, or at least damage, of the piezoelectric actuator.
In the conventional fuel injection device, the “voltage stroke” required for an actuation of the valve element, i.e., the difference between the initial voltage and the final voltage given a controlling of the piezoelectric actuator, increases given increasing fuel pressure acting on the valve element in the opening direction. Here, the fuel injection device is designed in such a way that, given a high fuel pressure, a large part of the available voltage stroke must be used up in order to open the valve element. After the opening, the valve element accelerates and moves until an equilibrium of forces prevails at the oppositely oriented pressure surfaces of the valve element. Given a high fuel pressure, this equilibrium point is not reached until the valve element is almost completely open.
Due to the described conditions, using the conventional fuel injection device it is difficult to inject very small quantities of fuel into a combustion chamber of an internal combustion engine. Such small injection quantities are desirable for pre-injections (pilot injection), for example.